FaceIndex is an index of a face in mesh.Faces.
When ComponentIndex refers to a vertex, any face that uses the vertex
may appear as FaceIndex. When ComponenctIndex refers to an Edge or
EdgeIndex is set, then any face that uses that edge may appear as FaceIndex.

Barycentric quad coordinates for the point on the mesh
face mesh.Faces[FaceIndex]. If the face is a triangle
disregard T[3] (it should be set to 0.0). If the face is
a quad and is split between vertexes 0 and 2, then T[3]
will be 0.0 when point is on the triangle defined by vi[0],
vi[1], vi[2], and T[1] will be 0.0 when point is on the
triangle defined by vi[0], vi[2], vi[3]. If the face is a
quad and is split between vertexes 1 and 3, then T[2] will
be 0.0 when point is on the triangle defined by vi[0],
vi[1], vi[3], and m_t[0] will be 0.0 when point is on the
triangle defined by vi[1], vi[2], vi[3].